1. Field of the Invention
The present disclosure relates to systems and methods for removal of contaminants from fluid streams. In particular, the present disclosure relates to systems and methods for the removal of volatile organic compounds from fluid streams.
2. Description of Related Art
The air within homes, office buildings, and other enclosed structures is often more polluted than outdoor air, especially in highly urbanized areas.
In some instances, the indoor air can include volatile organic compounds (VOCs) and/or semivolatile organic compounds (SVOCs). VOCs are organic chemical compounds with high enough vapor pressures, under normal conditions, to significantly vaporize and enter the atmosphere. SVOCs are organic compounds with typical vapor pressures between 10−2 and 10−8 kPa at room temperature, so that they exist both in the gas phase and the condensed phase. These organic compounds are emitted as gases from certain solids or liquids, including a number of items commonly found around a home or office, such as paint, furniture, building materials, office equipment, and cleaning supplies.
Buildings may utilize air purification systems to improve the quality of indoor air thus enabling the building operator to decrease ventilation and create an improved environment. The quality of indoor air is achieved through air purification using either aerosol removal or gaseous contaminant removal technologies. Photocatalysis is a proven technology for removal of gaseous airborne substances such as VOCs from the air supply. Photocatalytic air purifiers utilize a substrate or cartridge containing a photocatalyst, usually a titanium oxide based material, that interacts with airborne oxygen and water molecules to form hydroxyl radicals when placed under an appropriate light source, typically a UV light source. The hydroxide radicals then attack the contaminants and initiate the oxidation reaction that converts them into less harmful compounds, such as water and carbon dioxide. A commonly used photocatalyst is titanium dioxide, otherwise referred to as titania. Degussa P25 titania and tungsten dioxide grafted titania catalysts such as tungsten oxide on P25 have been found to be especially effective at removing organic contaminants under UV light sources.
One well known phenomenon that limits the effectiveness of photocatalytic oxidation (PCO) air purifiers is deactivation of the photocatalyst. Deactivation can occur reversibly or irreversibly and the extent and scope of the deactivation depends on the system configuration, including light intensity at the catalyst surface, the amount and configuration of the catalyst, and conditions in the ambient environment. Currently available systems have been found to exhibit a significant loss in catalytic ability when they are overwhelmed by organic contaminants such as ethanol, iso-propanol and other contaminants having a high affinity for the catalytic surface. Catalytic ability also decreases when the photocatalyst is challenged with a gaseous compound that, when oxidized, forms a compound or moiety that blocks an active site on the catalyst permanently unless acted on by an outside agent.
Compounds which contain only hydrogen, carbon and oxygen atoms usually only cause reversible deactivation, which only has a short term impact on the operation of the air purifier. However, if the VOC concentrations are high enough with the UV light on, or if the VOC concentrations last long enough with the UV light off, the photocatalysts are not able to interact with water to create sufficient hydroxyl radicals to keep active sights available for further reaction. The contaminants then may react with each other to form a varnish, which occupies the photocatalyst sites and blocks the ability of the photocatalysts to oxidize the contaminants. This or any type of permanent deactivation can lead to a significant expense for the operator of the air purification system due to the labor and equipment costs associated with cleaning and/or replacing the photocatalyst.
Another recently discovered deactivation route is caused by the mineralization of silicon-containing volatile compounds, especially the class of compounds known as siloxanes. Where as the aggregate amount of VOCs in air typically ranges from 100 to 1000 parts per billion by volume, siloxane concentrations can be two or more orders of magnitude lower. These siloxanes arise primarily from the use of certain personal care products or dry cleaning fluids, although they can also arise from the use of silicone caulks, adhesives and similar materials. However, when these silicon-containing compounds are oxidized, they form non-volatile silicon dioxide or hydrated silicon dioxide that deactivates the photocatalyst. This deactivation can be by one or more means such as relatively direct physical blocking of the active sites, blocking of the pores in the photocatalytic coating, or blocking the interaction of the VOCs with the active agent.
In the removal of contaminants from air, the oxidation of certain species can generate undesirable byproducts, or the contaminant itself can have a negative effect on the performance of the photocatalytic unit. Thus, UV/PCO air purification alone is not sufficient to reduce the concentrations of VOCs in the air within desired limits.